FIRST: REDUCE WEIGHT. If it doesn't have a purpose, it doesn't have a ride. Strip everything that you don't need, DO NOT bolt on a bunch of crap (like a wing) that will add weight esp. up high. Autocross tends to be lower speed and aero improvements do nothing below about 100 KPH except add weight. Reduce weight- it's all about the power to weight ratio.
I installed a subframe rigid collar kit and it has made my car a lot more responsive. Handling with the subframe collar kit is a lot better overall and it's a pretty cheap upgrade. they weight less than 1 lb.
For power, the intake is what you want to install first, but it should be last. Install a good header and exhaust first, then upgrade the intake. For a N/A engine I think IHE is the best place to start to increase power